MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 235 



It cannot fail to be observed that to a great extent the rate of 

 respiration is affected by the same circumstances as the frequency 

 of the heart (p. 107), and in the same direction. And, indeed, in 

 health, these two physiological quantities, amid all their absolute 

 variations, maintain to each other a fairly constant ratio (i to 4 or 



f _ -.._._ i to 5 in man). Even in many diseases 



this proportion remains tolerably 

 stable, although in others it is dis- 

 turbed. 



The total quantity of air expired, or, 

 what comes to the same thing, the 

 alteration in the capacity of the chest 

 during expiration, can be measured by 

 means of a gas-meter or of a spiro- 

 meter (Fig. 114), which consists of an 

 inverted graduated glass cylinder dip- 

 ping by its open mouth into water 

 and balanced by weights. The vessel 

 is sunk till it is full of water, the air 

 being allowed to escape by a cock. 

 The expired air is now permitted to 

 enter it through a tube, and displaces 

 some of the water. The spirometer 

 is adjusted so that the level of the 

 water inside and outside is the same, 

 and then the volume of air contained in it is read off. This gives 

 the volume of the expired air at atmospheric pressure. Similarly, 

 by breathing air from the spirometer the amount inspired can be 

 measured (p. 303). 



From 400 to 500 c.c. of air* are taken in and given out at each 

 respiration in quiet breathing. This is called tidal air. It amounts 

 to 35 pounds by weight 



I 



Fig. 114. Diagram of Spirometer. 

 A, vessel filled with water. B, 

 glass cylinder with scale C, 

 swung on pulleys and counter- 

 poised by weights W. D, tube 

 for breathing through. 



Vital 

 Copacit(j\ 



ear 



Tidal _.. 



Supplemental air 

 Residual air 



Fig. 115. Diagram to illustrate the Relative Amount 

 of Complemental, Tidal, Supplemental, and Residual 

 Air. 



in twenty-four hours, 

 or enough to fill, at 

 atmospheric pressure, 

 a cubical box with a 

 side of 8 feet. With 

 the deepest possible in- 

 spiration room can be 

 made for 2,000 c.c. more ; this is called complemental air. By a forced 

 expiration 1,500 c.c. can be expelled besides the tidal air ; and to this 



* The average for 81 healthy students, with an average body-weight of 

 66 kilos, was 460 c.c., or 7 c.c. per kilo. In 4 new-born children the tidal air 

 varied from 20 to 30 c.c., and from y6 to 7' 3 c.c. per kilo, which is not very 

 different from the amount in the adult. The pulmonary ventilation must 

 therefore be far more rapid in the child, since its respiratory frequency is so 

 much greater. 



